This invention relates to a signal-transmission system, and more particularly to a signal-transmission system capable of controlling errors for error-free transmission of static image signals.
Transmission of a static image by means of a telephone line system is expected to be realized in the near future because the existing telephone facilities are readily available for use. Particularly where transmission of a static image is undertaken through the telephone line system, there is the advantage of allowing said transmission to be carried out between any given localities only when so desired. For transmission of a static image of good picture quality through the telephone line system, however, it is necessary to apply digital transmission, shorten the time of transmission by using bit-reduction technique and applying high speed MODEM, and further suppress occurrence of errors resulting from the required high speed transmission of data signals with bit-reduction technique.
A static image transmission system includes a facsimile transmission system. This system carries out transmission of static images representing documents or bills between the transmitter and receiver through a transmission line. Application of, for example, a telephone line system for transmission of data signals of narrow frequency band enables the transmission area of an image to be more freely chosen over a broader range then when a private line is used for such transmission. However, application of the telephone line system has the drawback that random or burst errors are likely to arise in transmitted data due to appearance of noises on the transmission line or intrusion of noises thereinto from an external source. Known literature on such transmission errors and control thereof includes "Errors and error control" by H. O. Burton and D. D. Sullivan Proc. IEEE, vol. 60, No. 11, November, 1972. Further, an automatic repeat request (ARQ) system is already known as a signal transmission system capable of controlling errors. This ARQ system is set forth in "ARQ Error Control on the Satellite Channel" by A. G. Gatfield, IEEE International Communications Conf., 1974, pp. 22B-1 to 22B-5. This literature describes three types of the ARQ system, that is, "Stop and Wait ARQ", "Continuous ARQ" and "Selective Repeat ARQ" and the respective transmission efficiencies. It will be seen from this literature that the "Selective Repeat ARQ" system has the highest transmission efficiency. This "Selective Repeat ARQ" system is detailed in "A Selective Repeat ARQ System" by A. G. Gatfield and T. R. Dobyns, COMSAT Laboratories, IEEE, 1974 NTC, pp. 189 to 195. However, application of the "Selective Repeat ARQ" error control system for transmission of a static image would make it necessary not only to install a memory device having a sufficiently large capacity to handle an amount of data being transmitted between the transmitter and receiver, but also additionally to provide means for replacing (that is edit function means) previously supplied erroneous data by fresh error-free data. For this reason, the "Selective Repeat ARQ" error control system presents considerable difficulties in being applied for facsimile transmission of data through a telephone line system and consequently has not yet been put to practical use.
Where, with the "Selective Repeat ARQ" system, data transmission is controlled by dividing a transmission line into two different channels, that is, where a forward channel is used for data transmission, and a backward channel is used for data error control, then a data block is first sent forth through the forward channel from the transmitter to the receiver. In this case, the receiver issues an acknowledgement (ACK) control signal if data received is correct, and a negative acknowledgement (NAK) control signal if data received is erroneous, both through the backward channel. Where an error arises in either of the aforesaid control signals (ACK and NAK) during transit through the backward channel, then the transmitter supplies the receiver through the forward channel with an enquiry (ENQ) signal demanding retransmission of a control signal. At this time, the receiver again sends forth the control signal to the transmitter through the backward channel. Where the transmitter receives the retransmitted control signal in an error-free state, then the transmitter undertakes the succeeding data transmission, if the control signal is of the ACK type. If the control data is of the NAK type, then the transmitter supplies the receiver with a fresh error-free data block corresponding to the previously transmitted erroneous data block. Therefore, a check bit such as an error detection bit or error correction bit must be attached to an error control signal issued from the receiver to the transmitter through the backward channel. This eventually increases a total transmitted amount of control signals, causing completion of transmission of correct data to consume a great deal of time with the resultant decline in transmission efficiency.